Composite plastic closures, formed from suitable polymeric materials, have meet with widespread acceptance in the market place, these types of closures typically include an outer closure cap or shell, typically formed from polypropylene or other polymeric material, in an inter-sealing liner, typically formed from ethylene vinyl esotate (EVA) or other suitable material. Formation of these types of closures by a compression molding process has proven to be particularly commercially viable, permitting highly efficient formation of composite closures which can provide very good sealing performance such as on carbonated beverages or the like. U.S. Pat. No. 4,497,765, hereby incorporated by reference discloses techniques for compression molding of such composite closures, including compression molding of the outer closure cap, and compression molding of a sealing liner within the outer closure cap.
The compression molding process by which such closures are formed typically entails introduction of a pre-determined quantity of molten polymeric material into a compression mold, for closure cap formation, or into the outer closure cap itself for liner formation. For compression molding of the sealing liner, a quantity of molten, liner-forming plastic material, typically in the form of a pellet, is introduced into the outer closure cap, typically when the outer cap is in an inverted position, and the pellet of molten plastic material is positioned on an inside surface of a top wall portion of the outer closure cap. To facilitate formation of the sealing liner with the desired configuration to form a so-called top/side seal, including an inwardly facing sealing surface, the outer closure cap is formed with an annular skirt portion having an annular seal lip which is positioned in closely spaced relationship to the top wall portion of the closure cap, and defining an annular recess therewith.
During liner formation a liner-forming tooling assembly is inserted into the outer closure cap, and an outer sleeve of the assembly advanced to engage the annular seal lip of the outer closure cap. Thereafter, an inner plunger of the tooling assembly is advanced relative to the outer sleeve to compression mold the pellet of molten plastic material, thereby forming the sealing liner adjacent the top wall portion of the enclosure cap. The sealing liner includes a central, disc-shaped portion, and an integral annular sealing bead portion which is at least partially positioned within the annular recess of the outer closure cap.
Attendant to high-speed closure manufacture, including compression molding of sealing liners as described above, there can be problems with air becoming trapped in the region within which the sealing liner is being formed. Because the venting of the tooling assembly is provided at the interface between the tooling and the seal lip, air trapped within the annular recess beneath the seal lip cannot reach the vent and becomes trapped in the recess. As a consequence, the trapped air can undesirably result in air bubbles being formed in the sealing liner. Some of these air bubbles can be large enough to cause the seal to fail when the closure is applied to an associated container. Trapped air can extend as much as 45 degrees or more around the periphery of the closure. As the liner continues to form, it compresses this air pocket into one area, and when the tooling is no longer there to keep it compressed, it expands creating a large bubble that interferes with the sealing on an associated container.
Notably, the type and quantity of defects which can result from such trapped air is highly dependent upon the specific positioning of the molten plastic pellet on the inside surface of the top wall portion of the outer closure cap. If the pellet is in the very center of the top wall portion, a relatively large quantity of bubbles, and relatively large bubbles, are undesirably produced. As the location at which the pellet is position is moved away from the very center of the top wall portion, the resultant air bubbles are reduced in size and number, but the molding process results in a larger size of, and larger quantity of “knit” lines (where portions of the plastic material flow into each other), non-fills, and flash. There is typically no pellet location that results in no defects being formed. As a consequence, machine operators have found it advantageous to run equipment so that the molten pellet is positioned slightly off-center, thereby avoiding formation of excessively large air bubbles, while limiting formation of knit lines, non-fills, and flash. Significantly, the difference in pellet positioning between formation of acceptable air bubbles and non-fills and flash can be as little as 0.020 inches or 0.030 inches. Considering the high-speed operation of the machinery, such careful positioning of the pellet formation poses a very delicate “balancing act” to optimize the liner-forming process.
The specific configuration of the liner profile can influence formation of air bubbles. For some profiles, it is virtually impossible to eliminate air bubbles from closures being formed. For some different types of liner profiles, any where from 40% to 60% of the liners may exhibit air bubbles. Other profiles may exhibit air bubbles in 50% to 100% of the products being formed. While such air bubble formation is generally accepted as a “passable defect” it will be appreciated that in attempting to avoid “non-passable bubbles” the liner-forming process can be precariously close to producing liners exhibiting flash and non-fill portions.
It has been recognized that if the problem of air bubble formation could be eliminated, it would permit the liner-forming process to be performed by placement of the molten pellet in the center of the top wall of the closure, thus desirably acting to reduce other defects, such as knit lines, non-fills, and flash, and thus making the liner-forming process more robust and resistant to other variations in the process, including changes in the amount and type of liner-forming material, and changes in the liner profile.